Linear viscoelasticity,probe tack,and extrusion flow results of SEBS copolymers

Microphase‐separated poly(styrene‐b‐ethylene‐co‐butylene‐ b‐styrene) copolymers of different molecular weights with polystyrene (PS) volume fraction ? = 0.30 are investigated. Linear dynamic viscoelastic data reveal the blocking effect of the ordered PS microdomains, which produces a predominantly elastic response at low frequencies, associated with a mechanical relaxation. These results are correlated with tack and extrusion flow measurements, which involve polymer/metal adhesion processes. Nonlinearity implied in some experiments alters the ordered morphology of low molecular weight samples producing a viscous dominant behavior. Dissipation is reflected in viscous‐like adhesion and lack of viscoplastic response in extrusion flow experiments. For the highest molecular sample, only a rubbery or entangled state (G′ > G″ and G′ > 10⁵ Pa) is observed in the linear viscoleastic regime. This is associated with the lack of adhesion seen in both, probe tack experiments and capillary flow measurements. This lack of adhesion gives rise to a plug‐flow as the polymer slips in the capillary wall. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Investigation of concentration‐dependent diffusion on frontal instabilities and mass transfer in homogeneous porous media

Mass transfer plays an important role in influencing the efficiency of miscible displacements in solvent‐based processes in enhanced oil recovery. The mass transfer rate because of the pure molecular diffusion is very slow. However, this process can be greatly enhanced by the appearance of frontal instabilities called viscous fingering mechanisms, which are beneficial for improving the mixing and mass transfer between the injected solvent and oil. Instead of a piston‐like displacement, the interface between solvent and oil is very convoluted with intricate finger‐like patterns of the less viscous solvent intruding into the highly viscous oil. This intrusion significantly increases the surface area of contact of the two fluids and leads to more efficient mass transfer and mixing. Experimental measurements on the diffusion coefficients of two miscible fluids indicate that, instead of a constant diffusion coefficient (CDC), a concentration‐dependent diffusion coefficient (CDDC) is more realistic. In the present study, a CDDC relation in which the diffusion coefficient is exponentially proportional to concentration is adopted. Its effect on the development of frontal instabilities is examined through highly accurate nonlinear numerical simulations. The differences between the CDDC case and the widely assumed CDC case are discussed. Furthermore, the enhancement of frontal instabilities on mass transfer when the CDDC is considered is investigated at various mobility ratios and Peclet numbers. The special characteristics for the CDDC case indicate its important role in miscible displacements. Eventually, the relation of breakthrough time to parameters is correlated to accurately predict the breakthrough time in any CDDC scenario.     

Viscoelastic Liquids in Stirred Vessels – Part I: Power Consumption in Unaerated Vessels

Different shear‐thinning and elastic fluids (STE fluids) have been stirred under unaerated conditions, in vessels equipped with Rushton disc turbines. Their power consumption has been evaluated over a wide range of stirring rates and their Metzner‐Otto constant (k_s) has been measured. A correlation has then been proposed to predict k_s values for a Rushton turbine operating in non‐Newtonian solutions. Power curves of STE fluids have been drawn and compared with reference curves (Newtonian, shear‐thinning inelastic and elastic with constant shear viscosity fluids). The STE fluids have thus been divided into two categories. The STE fluids of the first category (STE I fluids), which are concentrated viscous solutions of polymers (guar, CMC) reducing the power consumption at the beginning of the transitional region and connecting with the Newtonian reference at higher Reynolds numbers. In contrast, STE solutions of the second category (STE II fluids), which are solutions of drag reducing polymers (PAA), are less viscous and more elastic. They reduce the power consumption at the end of the transitional region and do not connect with the Newtonian reference, at least until Re = 6000. A general correlation has finally been proposed to model the power curve of STE fluids stirred by a Rushton turbine from the laminar to the turbulent regions, as a function of their elasticity.     

Finite element simulation of three-dimensional viscoelastic planar contraction flow with multi-mode FENE-P constitutive model

Viscoelasticity is a characteristic of many complex fluids like polymer melts, petroleum, blood, etc. The investigation of viscoelastic flow mechanism has practical significance in both scientific and engineering field. Owing to strongly nonlinear, numerical method becomes a practical way to solve viscoelastic flow problem. In the study, the mathematical model of three-dimensional flow of viscoelastic fluids is established. The planar contraction flow as a benchmark problem for the numerical investigation of viscoelastic flow is solved by using the penalty finite element method with a decoupled algorithm. The multi-mode finitely extensible nonlinear elastic dumbbell with a Peterlin closure approximation (FENE-P) constitutive model is used to describe the viscoelastic rheological properties. The discrete elastic viscous split stress formulation in cooperating with the inconsistent streamline upwind scheme is employed to improve the computation stability. The numerical methods proposed in the study can be well used to predict complex flow patterns of viscoelastic fluids.     

Computational Modeling of Die Swell of Extruded Glass Preforms at High Viscosity

Computational simulations of glass extrusion are performed to quantify the effects of material behavior and slip at the die/glass interface on the die swell. Experimental data for three glass types are used to guide the computational study, which considers glass material to be viscous with and without shear thinning and viscoelastic using the Maxwell upper‐convected model. The study starts with assuming no‐slip at the glass/die interface to see if material behavior alone can explain the die swell results, and then considers slip using the Navier model where interface shear is directly proportional to the relative slip speed at the interface. Consistent with the possibility of slip and intended high viscosity applications, viscosity ranging from 10^7.4–10^8.8 Pa·s was used. Based on optimization of the various input parameters required to achieve the measured die swell and ram force values, the study concludes that interface slip occurred as only extreme values of the shear thinning parameters provided an alternative.     

Simulation of viscoelastic behavior of glassy polymers

The frequency master curves of thermoset and thermoplastic glassy polymers were determined by dynamic mechanical analysis, and the results were simulated by sinusoidal response of the standard viscoelastic model. The elastic and viscous elements of the model were determined and correlated with structure of glassy polymers. The advantage of this viscoelastic approach in toughening studies of polymers was discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 971–982, 1997     

Pressure drop in a split‐and‐recombine caterpillar micromixer in case of newtonian and non‐newtonian fluids

Microreactors are very promising tools for the design of future chemical processes. For example, emulsions of very narrow size distribution are obtained at much lower energy consumption than the one spent with usual processes. Micromixers play thereby an eminent role. The goal of this study is to better understand the hydrodynamic properties of a split‐and‐recombine Caterpillar micromixer (CPMM) specially with regard to handling viscoelastic fluids, a topic hardly addressed so far in the context of micromixers in general, although industrial fluids like detergent, cosmetic, or food emulsions are non‐Newtonian. Friction factor was measured in a CPMM for both Newtonian and non‐Newtonian fluids. For Newtonian fluids, the friction factor in the laminar regime is f/2 = 24/Re. The laminar regime exists up to Reynolds numbers of 15. For shear‐thinning fluids like Carbopol 940 or viscoelastic fluids like Poly Acryl Amide (PAAm) aqueous solutions, the friction factor scales identically within statistical errors up to a generalized Reynolds number of 10 and 0.01, respectively. Above that limit, there is an excess pressure drop for the viscoelastic PAAm solution. This excess pressure drop multiplies the friction factor by more than a decade over a decade of Reynolds numbers. The origin of this excess pressure drop is the high elongational flow present in the Caterpillar static mixer applied to a highly viscoelastic fluid. This result can be extended to almost all static mixers, because their flows are generally highly elongational. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2679–2685, 2013     

Flow Compartments in Viscoelastic Fluids Using Radial Impellers in Stirred Tanks

The influence of viscoelastic flow properties on fluid dynamics using radial impellers is investigated. The use of transparent model fluids allows for the optical measurement of general flow behavior with a fluorescence dying technique. By varying viscoelastic flow properties, size of agitators and rotational frequency, the impact of these parameters on fluid dynamics is analyzed. Toroidally shaped, cavern‐like flow compartments form around the agitators in all fluids in specific rotational frequency ranges, preventing an efficient mixing. By balancing elastic with centrifugal forces, a simple model is developed with which compartment sizes can be predicted with good accuracy. The results indicate a good suitability of the elasticity number as a scale‐up criterion.     

SLIP BOUNDARY CONDITION FOR VISCOUS FLOW OVER SOLID SURFACES

A slip boundary condition is derived for the flow of a viscous fluid over a solid surface, using the theory of thermal activation process. The slip velocity is proportional to the hyperbolic sine of the shear stress on the solid surface, and the slip boundary condition reduces to Navier's slip boundary condition for the flow of Newtonian fluids under small shear stresses. There exists a critical shear stress determining the onset of the slip flow.     

Profiles of Spreading Sessile Drops of Viscous Polymer Melts

The profile or shape of spreading drops of viscous polymer melts has always raised some questions regarding the basic forces inherent in and controlling the wetting phenomenon. The occurrence and nonoccurrence of “projecting feet” for spreading sessile drops has puzzled experimentalists for some time. Recent work for a homologous polymer series, differing only in molecular weight and molecular weight distribution and examined over a wide temperature range, has emphasized that an advancing projecting foot does occur and is dependent upon the molecular weight of the polymer material. The projecting feet can be interpreted as an effect resulting from the viscoelastic response of the material which can occur when the molecular weight of the polymer is higher than the characteristic critical molecular weight of entanglement of the material. For low molecular weight polymers the strong interfacial forces are not impeded by the bulk viscous response of the material whereas the present evidence indicates that, for high molecular weight material, the elastic or entangled component of the polymer may retard complete bulk flow or the redistribution of the polymer in harmony with the advancing interface. Previous inconsistencies in the literature regarding the observations of projecting feet may be explained by this model.     

Rheological examination of temperature dependence of conventional and polymer‐modified road bitumens

Rheological characterization of two types of road bitumens, conventional and polymer‐modified, has been examined in the temperature range between 20°C and 140°C. Tests were carried out before and after ageing following a thin‐film oven test. Polymer‐modified bitumens exhibit non‐Newtonian behaviour up to the 120°C due to a complex secondary structure formed by added polymers. For conventional bitumens, Newtonian behaviour was observed above 60°C. Special attention was paid to measurements and to analysis of the dynamic data of oscillatory shear. The mechanical spectra in a wide frequency range have been obtained using the WLF time‐temperature superposition principle. The analysis of viscoelastic data clearly showed the differences between the two types of bitumen. Conventional bitumens were more sensitive to temperature and to the ageing effects. For polymer modified bitumens, the elastic contribution to viscoelastic response was more pronounced, and independent of temperature and ageing.     

Dynamic and capillary rheology of LDPE‐EVA–based thermoplastic elastomer: Effect of silica nanofiller

The effect of pristine silica nanoparticles on the dynamic and capillary rheology of a model LDPE‐EVA thermoplastic elastomeric system is explored in this paper. The pristine silica nanoparticles were melt‐blended with the LDPE‐EVA system at 1.5, 3, and 5 wt% loadings, respectively, by varying the sequence of addition. In one of the compositions, coupling agent bis‐[3‐(triethoxysilyl)propyl] tetrasulphide (Si‐69) was used to improve the interaction of hydrophilic silica particles with polymer matrix. Results obtained reveal that the viscoelastic behavior of such composites is influenced remarkably by loadings of silica, variation of sequence, and addition of Si‐69. Upon addition of coupling agent, G′ value increases especially at higher strain levels due to increased polymer‐filler interactions. All systems with various loading of nanosilica represent an increase in elastic response with increasing frequency. Both the unfilled and filled blends exhibit rheological behavior of non‐Newtonian fluids. But interestingly, the viscoelastic response varies markedly with the temperature. The dynamic and steady shear rheological properties register a good correlation in regard to the viscous vs. elastic response of such systems. Finally, the rheological behavior is correlated with morphology of the present system processed at various shear rates. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Autonomous damage initiated healing in a thermo‐responsive ionomer

The partially neutralised poly[ethylene‐co‐(methacrylic acid)] copolymer Surlyn 8940^® (DuPont) ionomer exhibits damage‐initiated healing during high‐energy impact. This is attributed to the hierarchical structure of ionomers, arising from the presence of ionic aggregates and hydrogen bonding. This work investigates the mechanism of this process using novel techniques developed here. The ionomer's response to penetration has been found to consist of three consecutive events: an initial elastic response, an anelastic response and pseudo‐brittle failure. In addition, the ultimate level of healing has been shown to be dependent upon the elastic response during impact as well as post‐failure viscous flow. Increasing the local temperature at impact consistently increases elastic healing, although further improvements in healing are minor once the local temperature increases beyond the melting point. Below the order‐to‐disorder transition, microscopic investigations reveal severe plastic deformation while the lack of shape memory reduces the comparative level of elastic healing. Above this temperature, healing is facilitated by elastomeric behaviour at the impact site, while above the melting point a combination of elastomeric and viscous flow dominates. This work provides for the first time evidence of the consecutive healing events occurring during high‐impact penetration for ionomers. The hierarchical structure of ionomers and its impact upon the microstructure have been shown to be critical to the process. Comparison of the mechanical response during impact with that of non‐ionic polymers further highlights this. In addition, slow relaxational processes occurring post‐impact are found to facilitate further recovery in mechanical properties. Copyright © 2010 Society of Chemical Industry     

Creep and stress relaxation modeling of vinyl ester nanocomposites reinforced by nanoclay and graphite platelets

This article discusses the viscoelastic behavior of a vinyl ester (Derakane 411‐350) reinforced with 1.25 and 2.5 wt % nanoclay and exfoliated graphite nanoplatelets during short‐term creep and relaxation tests with a dynamic mechanical analyzer. Linear viscoelastic models are generally composed of one or more elements such as dashpots and springs that represent the viscous and elastic properties. Stress relaxation data from the dynamic mechanical analyzer have been used to obtain the elastic parameters based on model constitutive equations. The standard linear solid model, which is a physical model, has been used for predicting the creep deformation behavior of the vinyl ester nanocomposites over a wide temperature range. Some correlations have been made with the mechanical model, such as the effect of temperature on the deformation behavior, which is well explained by the dashpot mechanism. At lower temperatures, higher creep compliance has been observed for the vinyl ester versus the nanocomposites, whereas at temperatures near the glass‐transition temperature of the vinyl ester, creep compliance in the nanocomposites is closer in magnitude to that for the vinyl ester. The creep response of the pure vinyl ester and its nanocomposites appears to be modeled reasonably well at temperatures lower than their glass‐transition temperatures. A comparison of the predictions and experimental data from the creep tests has demonstrated that this model can represent the long‐term deformation behavior of these nanoreinforced materials reasonably well. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Measurements of the viscoelastic moduli of an acrylate polymer in bulk and film form using a contact method

This paper describes a new method for the measurement of the viscoelastic properties of polymers using a contact mechanics approach. The latter is based on the determination of the tangential response of a macroscopic contact between a polymer specimen and a rigid sphere under small amplitude cyclic micro-motions. Using an acrylate polymer, it was found possible to achieve contact conditions where the tangential behaviour of the contact is strictly linear. Under such conditions, the measurement of the contact stiffness allowed to determine the viscoelastic moduli of the bulk polymer through the glass transition zone. In addition, it was also found to be possible to measure the damping properties of thin films (30 μm) using the same method. The results indicated a shift of the glass transition temperature of the films as compared to bulk specimens. This result was interpreted as an indication of the sensitivity of the glass transition of amorphous polymers to the hydrostatic pressure.     

Flow of viscoelastic surfactants through porous media

We compare the flow behavior of viscoelastic surfactant (VES) solutions and Newtonian fluids through two different model porous media having similar permeability: (a) a 3D random packed bed and (b) a microchannel with a periodically spaced pillars. The former provides much larger flow resistance at the same apparent shear rate compared to the latter. The flow profile in the 3D packed bed cannot be observed since it is a closed system. However, visualization of the flow profile in the microchannel shows strong spatial and temporal flow instabilities in VES fluids appear above a critical shear rate. The onset of such elastic instabilities correlates to the flow rate where increased flow resistance is observed. The elastic instabilities are attributed to the formation of transient shear induced structures. The experiments provide a detailed insight into the complex interplay between the pore scale geometry and rheology of VES in the creeping flow regime. © 2017 American Institute of Chemical Engineers AIChE J, 64: 773–781, 2018     

Effects of swelling on the viscoelastic properties of polyester films made from glycerol and glutaric acid

Viscoelastic properties have been determined for poly(glycerol‐co‐glutaric acid) films synthesized from Lewis acid‐catalyzed polyesterifications. The polymers were prepared by synthesizing polymer gels that were subsequently cured at 125°C to form polymer films. The polymers were evaluated for the extent of reaction before and after curing by Fourier transform infrared spectroscopy. They were subsequently immersed in dimethylsulfoxide, tetrahydrofuran, water, methanol, and hexane for 24 h. The amounts of solvent absorbed were monitored and recorded. Dependent up the solvent used, the polymers were able to absorb 9.5–261% of its weight. The effects of the solvent absorption on the viscoelastic properties of the polyester films were evaluated by determining their elastic modulus (G′), viscous modulus (G″), tan δG″/G′, and complex viscosity (η*) by performing oscillatory frequency sweep experiments. The elastic modulus (G′) and viscous modulus (G″) were both higher for the dry polymers than the solvent‐absorbed polymers. However, the polymer films were all higher in elastic (G′) character than viscous (G″) character. Therefore, tan δG″/G′ < 1 before and after immersion in solvents. Values for η* decreased with angular frequency for all of the polyesters tested in this study. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Characterization of viscoelastic fluid flow in a periodically driven cavity: Flow structure,frequency response,and phase lag

Transport phenomena in periodically driven cavities are important because of their relevance in polymer processing and microfluidics. The transient periodic flow of viscoelastic fluids in a cuboidal cavity, with periodic motion of top plate, was studied in this work. Flow with a characteristic time scale was achieved through the simple harmonic motion of the top plate. The flow in the cavity was characterized by measuring planar velocity fields using particle image velocimetry (PIV). Temporal variation of velocity except at central vertical plane showed predominance of the plate frequency. The temporal point variations, though seemingly similar to those for Newtonian and purely viscous non‐Newtonian fluids, led to rich varieties of spatial flow structures in case of the viscoelastic fluids. The overall flow behavior was characterized using spatial variations, phase trajectories, and streamline patterns. The transition from low Reynolds number steady‐lid driven type flow to complex vortical patterned flow was observed during a cycle of periodic motion of viscoelastic fluids. The effects of elasticity and inertia on the flow fields were analyzed. Computational fluid dynamics simulations with purely viscous shear thinning fluid (power law) and Newtonian fluid showed significant differences with experimental measurements on viscoelastic fluids. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Rheological studies of mixed printing pastes from sodium alginate and modified xanthan and their application in the reactive printing of cotton

The rheology of printing pastes using sodium alginate and modified xanthan as mixed thickeners has been measured by steady shear tests and dynamic strain sweep tests. The rheological results showed that, with a small addition of modified xanthan to sodium alginate, the flowability of the mixed printing pastes was better than that with sodium alginate alone. Subsequently, as the addition of modified xanthan increased, the apparent viscosity at low shear rates increased gradually, and the mixed printing pastes gained increasingly pronounced shear‐thinning features. In addition, the mixed printing pastes with more sodium alginate exhibited mainly viscous behaviour under strain, and the liquid‐like features became increasingly weak with the addition of modified xanthan. On the other hand, the mixed printing pastes with more modified xanthan exhibited mainly elastic behaviour within the linear viscoelastic region, and the solid‐like features became increasingly marked with the addition of modified xanthan. Mixtures of sodium alginate and modified xanthan can be used as thickeners in the reactive printing of cotton, producing a good colour yield, levelness, and outline sharpness. In particular, for large patterns, the mixed printing paste performed best when the ratio of sodium alginate to modified xanthan was 80:20; for fine patterns, it performed best when the ratio of sodium alginate to modified xanthan was 20:80.